Of the various rays of the sun, having different wavelengths, an ultraviolet ray with a wavelength of 150 to 380 nm functions to discolor a substance or to damage a skin, and an infrared ray with the wavelength of 780 to 2300 nm has heat energy corresponding to 53% of solar energy. Accordingly, there is a need to develop functional coating material (blocking material) capable of allowing a visible ray with the wavelength of 380 to 780 nm to easily transmit therethrough and effectively blocking the ultraviolet and infrared rays.
A conventional sunlight blocking film for automobiles is disadvantageous in that it cannot be applied to the front windows of automobiles, through which heat is mostly transmitted, because of its poor transparency even though it can block the ultraviolet ray. Additionally, the conventional sunlight blocking film cannot effectively block heat outside the automobiles, and thus, air conditioners of the automobiles are excessively used to cool insides of the automobiles in summer, leading to enormous energy consumption and serious pollution. In winter, indoor heat is mostly lost through windows, which brings about energy waste. To avoid heat dissipation, a transparent heat blocking film may be attached to the windows.
In order to better understand the background art of the present invention, a description will be given of a conventional transparent coated film capable of allowing the visible ray to easily transmit therethrough and blocking the infrared ray. Technologies regarding the conventional transparent coated film may be classified into a) a gas phase process, in which an indium tin oxide (hereinafter, referred to as “ITO”) film is formed according to physical and chemical deposition processes or a sputtering process, and b) another process using anthraquinone-, naphthalocyanine-, cyanine-, phthalocyanine-, metal complex-, diammonium-, azo compound-, copper compound-, polymethine-, triphenylmethane-, and quinone-based pigments.
In detail, in the case of the gas phase process (a), it is necessary to use a high-priced sputtering device requiring high vacuum and high precision, and thus, the gas phase process is disadvantageous in terms of the production costs and productivity. Furthermore, the process of b) is disadvantageous in that sufficient infrared blocking effect is not ensured at a relatively wide wavelength range but only the infrared ray with a specific wavelength is blocked, a surface of the coated film is discolored due to UV, heat, moisture and the like, and an infrared blocking ability is not constantly secured.
To avoid the above disadvantages, there has been developed a composition for a near-infrared blocking filter, which is mass-produced at relatively low cost, and which includes ITO ultrafine powder or antimony tin oxide (hereinafter, referred to as “ATO”) ultrafine powder, metal oxides, and organic and inorganic dyes and pigments (JP-A-7-24957, JP-A-7-70363, JP-A-70482, and JP-A-7-445).
However, the composition including the ITO and ATO ultrafine powders is disadvantageous in that the transmissivity is relatively low within a near-infrared range with the wavelength of 1400 nm or more, but the transmissivity is relatively high within the near-infrared range with the wavelength of 701 to 1399 nm, and thus, it is difficult to secure a desirable infrared blocking effect.
Furthermore, in case that a metal oxide and a metal form a multilayered structure according to the sputtering process, even though an infrared blocking effect is improved, the productivity is poor, the production costs are relatively high, and the multilayered structure reflects sunlight, bringing about a dazzling phenomenon. In addition, the multilayered structure leads to a wave phenomenon in night, thereby hindering any view. As well, it is easily corroded in a zone, at which air contains a great amount of salt.
Recently, many studies have been made to develop a process using near-infrared absorbing dyes and pigments. However, this process has disadvantages in that sufficient infrared blocking effect is not ensured at the relatively wide wavelength range but only the infrared ray with the specific wavelength is blocked, the surface of the coated film is discolored due to UV, heat, moisture and the like, and the infrared blocking ability is not constantly secured.
Additionally, a glass filter, on which a metal film is deposited, and a phosphate glass filter, containing metal ions, are well known to those skilled in the art. However, these glass filters are insufficiently competitive in terms of production costs.
Generally, a process of producing the ITO powder includes reacting an aqueous solution, containing indium (In) and a small amount of tin (Sn), with alkali to co-precipitate indium and tin hydroxides, and heating and sintering the indium and tin hydroxides under atmospheric air to produce oxide compounds. However, the ITO powder thusly produced has a disadvantage in that it does not efficiently block the infrared ray even though the ITO powder has excellent transparency within a visible ray wavelength range, because excellent infrared blocking efficiency of the ITO powder is ensured at the wavelength range of 1000 nm or more.
Furthermore, it is widely known that a color of the ITO powder is yellow when it is oxidized, and is blue when it is partially reduced. Accordingly, when the ITO powder, including particles with a particle size of 100 nm or less, is dissolved in a coating liquid and then coated on a base, the coated surface is not transparent but has an opaque white color. Hence, it is difficult to apply the ITO powder to the transparent base.
Therefore, there remains a need to develop the ITO powder having an excellent infrared blocking effect. With respect to this, a conventional method of producing an infrared blocking ITO powder is disclosed in Korean Pat. Publication No. 01-0214428, in which a co-precipitate of indium and tin is sintered under a pressurized inert gas.
However, the above conventional method is disadvantageous in that production costs are relatively high, explosion easily occurs, and productivity is poor because the ITO powder is produced at a relatively high pressure of 5 to 60 kgf/cm2.
Other disadvantages of the conventional method are that when only the partially reduced ITO powder is used, a chromaticity coordinate haze problem occurs and the ITO powder is easily oxidized under atmospheric air.
In the case of using only ATO, when the infrared blocking ability is improved, the powder has very low transparency within the visible ray wavelength range, and thus, it is practically difficult to commercialize the powder.
Further, in the case of using a powder mixture, containing ITO and ATO powders mechanically mixed with each other, when a sufficiently many amount of ATO powder is used to compensate for problems caused by the ITO powder, the infrared blocking ability is reduced and the transmissivity is reduced at the visible ray wavelength range. On the other hand, when a small amount of ATO powder is mixed with the ITO powder, it is impossible to overcome the problems, such as the chromaticity coordinate haze problem and the oxidation of the ITO powder under atmospheric air, occurring in use of the ITO powder.